FIG. 1 is a schematic diagram showing the configuration of a system including a wireless terminal and access points.
According to FIG. 1, access points 1 are connected to the Internet through access networks while they communicate with a wireless terminal 2 through the air. A wireless terminal 2 staying in an area of WLAN configured around an access point 1 is allowed to be connected to the Internet (a host network) via the access point 1. The wireless terminal 2 shown in FIG. 1 is in a position where the terminal can communicate with three access points 1. As an example, the wireless terminal 2 discovers an objective access point AP2 to be connected with the terminal, then uses the discovered access point AP2 in order to be connected to the Internet via the access point AP2.
WLANs are generally configured to adopt techniques working in MAC (Media Access Control) layer to control packet transmission between wireless terminals and access points. The structure of MAC frames exchanged between wireless stations via the MAC layer is prescribed, for example, in the IEEE802.11 standard.
Infrastructure mode specified in the IEEE802.11 standard provides two kinds of scan method, “active scan method” and “passive scan method”, for a wireless terminal discovering an access point in a WLAN. The “active scan method” is a method in which a wireless terminal broadcasts a probe request toward access points and then receives a probe response from each access point to thereby discover the access point to be connected with the terminal. On the other hand, the “passive scan method” is such that an access point broadcasts beacons at regular intervals and a wireless terminal receives the broadcasted beacon to thereby discover the access point to be connected with the terminal. Note that the beacons, the probe requests and the probe responses are kinds of control signals for communication management which are exchanged between access point(s) and wireless terminal(s).
Generally, wireless terminals are equipped with both functions of performing active scan method and passive scan method. The wireless terminal, when discovering an objective access point by either of these two methods, performs a processing sequence for connection with the objective access point to be connected.
FIG. 2 is a sequence diagram showing a sequence based on the active scan method.
As shown in FIG. 2, first, a wireless terminal 2 transmits a probe request in a broadcasting way. During such frame-broadcasting, the wireless terminal 2 may consume more electric power. The probe request broadcasted by the wireless terminal 2 is received by all the access points 1 (AP1, AP2 and AP3) which employ common channels and are located in a range where the electric wave of the probe request can reach.
In response to receiving the probe request, the respective access points 1 send probe responses to the wireless terminal 2. In transmitting the probe responses, access points 1 control the timing of transmission by means of the techniques working for the MAC layer of WLAN so that the sent probe responses do not collide with each other. For example, there adopted is a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) method in which a plurality of terminals can establish communication sessions by making mutual concessions in terms of timing when obtaining a common WLAN channel to be used.
Meanwhile, the wireless terminal 2 waits (stands by), during a comparatively short time, for a probe response with comparatively small power consumption. When receiving within the waiting time a probe response sent from the objective access point AP2 to be connected, the wireless terminal 2 carries out a connection sequence with the access point AP2.
FIG. 3 is a sequence diagram showing a sequence based on the passive scan method.
As shown in FIG. 3, first, access points 1 transmit beacons at regular intervals in a broadcasting way. Meanwhile, a wireless terminal 2 waits (stands by), using comparatively small power consumption, for a beacon during a comparatively long time equal to or longer than the transmission period of the beacons. When receiving within the waiting time a beacon sent from the objective access point AP2 to be connected, the wireless terminal 2 carries out a connection sequence with the access point AP2.
The access points 1 broadcast the beacons regularly (for example, at an interval of about 100 milliseconds) in, e.g., the 2.4 GHz band. The wireless terminal 2 then tries to sense (searches for) a beacon in each of, e.g., ten channels of the 2.4 GHz band, the beacon being broadcasted from the access point that is a connection party. Here, the wireless terminal 2 has to await the beacon for 100 milliseconds (msec) or more in every channel. This results in requiring the waiting time of about 1 second calculated as follows for searching all the channels.100 msec×10 channels=1 secIn the case where the wireless terminal 2 is a battery-driven portable-phone or smartphone, it is critical from a viewpoint of suppressing electric power consumption to shorten the waiting time as much as possible.
As explained above, the active scan method realizes a comparatively short waiting time for receiving probe responses though requiring a temporal increase in power consumption when sending the probe request. On the other hand, the passive scan method needs a comparatively long waiting time for receiving beacons although not requiring so much electric power. Accordingly, the active scan method, which is able to discover an access point in a comparatively short time, achieves smaller total power consumption than that of the passive scan method. Therefore, wireless terminals generally adopt the “active scan method” that consumes comparatively small power and discovers an access point in a comparatively short time. When entering the area of an access point to be connected, the wireless terminal receives a probe response replied in response to a probe request transmitted by the terminal in the active scan method, to thereby discover the target access point.
As an example of such a conventional technology, Patent Document No. 1 discloses a technique in which, when trying to discover an access point, a wireless terminal adopts respective active scan method and passive scan method selectively based on whether the terminal is inside or outside the communication range.